Two-photon (vacuum ultraviolet + visible) spectroscopy of argon, krypton, xenon, and molecular hydrogen

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Abstract
Two‐photon transitions have been examined in argon, krypton, xenon, and molecular hydrogen using a coherent vacuum ultraviolet (VUV) photon at a fixed wavelength of 118 nm and a tunable photon from a dye laser. The 118 nm VUV photon is produced by third harmonic generation in xenon using UV light at 355 nm from a frequency tripled Nd:YAG laser. Though the VUV intensity is very weak, (∼100 nJ per pulse) it was utilized very efficiently since most of VUV photons in the ionization region were absorbed. Spectra were obtained in the region of 12.5 to 13.5 eV and the corresponding two‐photon rate constants were calculated. This is the first time that coherent VUV light has been employed with tunable visible light for the production of two‐photon spectra and the measurement of two‐photon rates. The two‐photon ionization rate of xenon was measured using photons at fixed wavelengths of 118 and 355 nm. A new parameter is proposed for direct comparison of the data from various two‐photon experiments.